There is presently a trend in the electronics industry towards achieving an ever greater density of circuits within a given area on a semiconductor chip, which has led to an increased number of more closely spaced input/output connections to the chip. When the semiconductor chip is packaged, each input/output connection on the chip is coupled to a lead or contact pad (hereinafter collectively referred to simply as "leads") on the package so that as the number of input/output connections increases, so too do the number of leads. To avoid increasing the package size as the number of leads increase, the size of and the spacing between leads are reduced. Presently, direct soldering or lead attachment (wire bonding) remains an effective technique for bonding the leads on a semiconductor chip package to a set of metallized areas on a circuit board. Such techniques are also employed for directly attaching a chip to a substrate such as a circuit board. However, as the lead pitch continues to decrease, soldering or lead bonding may no longer be feasible, and other interconnection techniques will be necessary.
Recently, polymer interconnect structures, comprised of electrically conductive particles mixed within an insulative polymer matrix, have shown much promise as an alternative interconnection technique to soldering. The particles, typically, silver-plated nickel or glass spheres, are dispersed or arranged in the polymer matrix prior to curing to create a plurality of laterally spaced conductive paths, each extending through matrix in the z direction. These conductive paths in the z direction are electrically insulated from each other and thus allow the polymer interconnect structure to exhibit anisotropic conductivity.
Presently there are two types of polymer interconnect structures. The first type of polymer interconnect structure, known as a "conductive polymer interface" (CPI) structure, is characterized by a polymer matrix fabricated from an elastic material, such as a silicone or the like. The conductive spheres in the CPI-type interconnect structure are aligned in lateral chains or columns which extend in the z direction, thus affording the structure its anisotropic conductivity.
The other type of interconnect structure, known as a "direct chip attach" (DCA) structure, is characterized by a polymer matrix fabricated from a thermoset or thermoplastic material, typically an epoxy, which is adhesive. Unlike the CPI interconnect structure, which contains chains of conductive spheres, the spheres within the DCA-type structure are arranged in a monolayer so that a portion of each sphere has a portion thereof proximate a major surface of the polymer matrix. As their name suggests, the DCA interconnect structures are primarily intended for directly attaching a semiconductor chip to a major surface of a substrate (i.e., a circuit board) so that each conductive pad on the chip is in electrical contact with a metallized area on the board.
With the present day CPI- and DCA-type interconnect structures, the conductivity is achieved by mechanical contact with the conductive spheres. This type of conductivity is generally effective because the polymer matrix tends to have a significantly higher coefficient of expansion than the conductive spheres. Hence, the conductive spheres are forced into contact with each other, when, in the case of the CPI-type structure, the structure is clamped between a pair of conductive members. In the case of the DCA-type structure, the adhesive properties of the matrix serve to hold the spheres in contact with the conductive members to be interconnected thereby.
However, the effectiveness of the electrical connection achieved by the mechanical contact of the spheres tends to degrade when there are temperature variations. Further, this type of electrical connection is susceptible to contamination and corrosion which is undesirable, especially with semiconductor chips which have an extreme sensitivity.
Thus, there is a need for a conductive polymer interconnect structure capable of providing a reliable bond notwithstanding temperature variations.